The present invention relates to a sealed prismatic battery in which an abnormal increase in internal pressure of the battery would cause an opening vent to cleave and would thus result in release of gas from within the battery.
In a sealed prismatic battery such as primary battery using lithium metal as active material and secondary battery undergoing charge and discharge with use of lithium ion, when an excessive electrical load is applied or an excessive thermal load is applied, a short circuit occurs within the battery thus generating gas, and an internal pressure of the battery abnormally increases. When the battery is overcharged, gas is generated within the battery due to decomposition of electrolyte and thus the internal pressure of the battery increases abnormally.
When the battery can no longer withstand the abnormal increase in the internal pressure, the battery casing would explode and contents thereof, such as electrolyte, would be spattered. Therefore, a battery casing is provided with an opening vent as disclosed in various patent documents such as JP H11-185714 A (FIGS. 2, 4), JP 2000-285892 A (FIG. 2), JP 2001-143664 A (FIGS. 1-6), JP 2001-266804 A (FIGS. 2-10), JP 2001-345083 A (FIGS. 2-20), JP 2002-25525 A (FIGS. 1, 2), JP 2003-297322 A (FIGS. 3, 6-8), JP 2004-39294 A (FIGS. 3-9), JP 2004-79330 A (FIGS. 3, 7-10), JP 2005-108584 A (FIGS. 1, 6-8), and JP 2005-38773 A (FIGS. 3, 5). With an abnormal increase in internal pressure of the battery, deformation of the battery casing causes cleavage in the opening vent, and gas in the battery is released through the cleavage.
In a sealed prismatic battery used in compact electronic apparatus such as portable telephone, a battery casing is shaped like an oval quadrangle having a width in a direction of thickness smaller than a height in a longitudinal direction and than a length in a lateral direction. With an abnormal increase in internal pressure of the battery, center parts of side walls of the battery casing that face in the direction of thickness expand outward, and side walls of the battery casing that face in the lateral direction and walls thereof that face in the longitudinal direction deform so that the central parts thereof are recessed curvedly.
The applicant thought that provision of the opening vent at a site on the battery casing prone to great deformation xvas preferable in terms of reliable cleavage action, and examined which was the best place to be provided with the opening vent among the side walls facing in the lateral direction, the side walls facing in the direction of thickness, and the walls facing in the longitudinal direction on the battery casing. Though provision of the opening vents on the side walls facing in the direction of thickness or the wails facing in the longitudinal direction on the battery casing is disclosed in the above patent documents, it has been found that the deformation in the side walls facing in the direction of thickness and the walls facing in the longitudinal direction is not so great. In accordance with the above patent documents, that is, reliable cleavage action of the opening vent cannot be assured and reliability of the opening vent is thus poor.
In the sealed rectangular oval battery in which the side walls of the battery casing that face in the lateral direction are generally shaped like circular arcs swelling outward in the lateral direction, strengths of the side walls facing in the lateral direction are increased and thus the side walls resist being deformed. When the internal pressure of the battery increases abnormally, accordingly, stresses are concentrated in vicinity of upper and lower ends of the side walls of the battery casing facing in the lateral direction and the upper and lower ends are greatly deformed.